Permanent magnet materials for electric motorsAmy J. Born | June 10, 2020
The demand for permanent magnets in electric motors is growing, and designers need to understand the benefits and tradeoffs of the high energy magnet materials available today in order to find the best fit for their applications.
The four primary groups of high energy magnets are aluminum nickel cobalt (alnico), ferrite (also known as ceramic), samarium cobalt (SmCo) and neodymium iron boron (NdFeBo). The important characteristics of permanent magnets are mechanical strength, cost, performance at different operating temperatures and energy product.
Alnico is the mechanically strongest material, ferrite is the lowest cost, SmCo has the highest operating temperature and NdFeBo has the highest energy material. NdFeBo and SmCo are the two most commonly used in electromechanical devices today.
NdFeBo magnets are available in grades ranging from N through EH which correspond to their material property characteristics of temperature, residual induction and maximum energy product or BH(max). The energy product for this type of magnet corresponds with temperature. The higher the temperature, the lower the energy product. EH grade works best at elevated temperatures, as high as 200° C, but the BH(max) is only 28 to 35 MGOe (Mega-Gauss•Oersted).
The N grade has a BH(max) of 38 to 52 MGOe, the highest energy product of any permanent magnet category available, but only at a temperature of 80° C. As a result, NDFeB is well-suited to use in room temperature motor applications, sensors and other electronics, various electromechanical devices and revitalizing legacy platforms. Its benefits include increased power for low-temperature applications, maximizing torque and acceleration, less material (footprint reduction), more energy, greater holding strength and stronger magnetic filtration attraction. One downside is that NdFeB requires plating to protect it from oxidation and degradation.
SmCo has two compositions (2:17 and 1:5), as well as various grades. It has the highest operating temperature of any permanent magnet type, and the lowest risk of demagnetization. The UHT grade can operate at up to 550° C without the risk of demagnetizing. This makes it highly valuable for demanding applications such as guidance systems, gyroscopes, traveling wave tubes, and other products that need constant magnetic field strength. The highest energy SmCo has 34 BH(max) typically and operates at 300° C. SmCo harnesses more energy and increases application power to output. Higher energy equals higher output, rotor torque, sensor strength and magnetic coupling torque. SmCo uses less material so the higher grades can reduce overall weight and, consequently, material cost. SmCo, therefore, is ideal for defense, commercial aerospace motors, microwave, traveling wave tubes, medical instrumentation and minimally invasive medical applications. SmCo does not require surface plating below 400° C.
Ultimately, when choosing the high energy magnet material for an electric motor application, two key tradeoffs are temperature versus energy product and plating versus no plating.
Learn more about selecting permanent magnet materials and rotor design considerations for electric motors in this free on-demand webinar presented by Electron Energy Corporation.